Metabolic Reactions and Enzymes

Energy and Thermodynamics

• Energy: Ability to do work or bring about change. It exists in various forms and is essential for all living organisms.

• Kinetic Energy: Energy of motion, such as mechanical energy. Examples include moving objects, waves, and molecules.

• Potential Energy: Stored energy that can be converted into kinetic energy. Examples include chemical bonds, concentration gradients, and electric potential.

• First Law of Thermodynamics (Law of Conservation of Energy): Energy cannot be created or destroyed, only transformed from one form to another. The total energy in an isolated system remains constant.

• Second Law of Thermodynamics: Energy transformations increase entropy (disorder). During energy conversions, some energy is always lost as heat, increasing the disorder of the system.

• Living organisms require a constant supply of energy (e.g., solar energy) to maintain order and carry out life processes.

Energy Transfer and Entropy

• Systems move from organized states (more potential energy, less stable) to less organized states (less potential energy, more stable). This is crucial for understanding how energy is used in biological systems.

• Disorder is more probable than order. Entropy is a measure of this disorder, and it tends to increase over time in a closed system.

• Energy transfers constantly occur in everyday activities, increasing entropy. No energy transfer is 100% efficient; some energy is always lost as heat.

Types of Reactions

• Reactions are divided into those requiring energy input and those releasing energy, which is fundamental to understanding metabolism.

• Endergonic Reactions: Require energy input; positive \Delta G (change in free energy) (e.g., photosynthesis). These reactions are non-spontaneous and require energy to proceed.

• Exergonic Reactions: Release energy; spontaneous; negative \Delta G. These reactions are spontaneous and release energy.

Enzymes

• Enzymes: Biological catalysts that speed up chemical reactions by lowering the activation energy. They are essential for life processes.

• Substrates: Reactants in an enzymatically catalyzed reaction. Enzymes bind to substrates to facilitate chemical reactions.

• Each enzyme accelerates a specific reaction; metabolic pathways require specific enzymes. This specificity is crucial for regulating metabolic processes.

• Genetic mutations in enzymes can cause diseases due to altered enzyme function or activity.

Enzyme Function

• Active Site: Area on the enzyme where the substrate binds. This is where the chemical reaction occurs.

• Lock and Key Model: Describes the specific fit between enzyme and substrate, highlighting the specificity of enzyme-substrate interactions.

• Induced Fit: Enzyme changes shape when the substrate enters the active site to optimize binding and facilitate the reaction.

• Enzymes lower the energy of activation, facilitating reactions, making them occur faster than they would without a catalyst.

Enzyme Mechanisms

• Enzymes can break down or synthesize molecules, playing a critical role in metabolic pathways.

• Degradation: Enzyme breaks down a molecule into two product molecules (hydrolysis may be involved). This is essential for digestion and breaking down complex molecules.

• Synthesis: Enzyme joins two substrate molecules, releasing a simple product (dehydration synthesis may be involved). This is crucial for building complex molecules from simpler ones.

Energy of Activation

• Energy of Activation: Energy needed to start a chemical reaction. Enzymes affect this but not free energy change.

• Enzymes lower the energy of activation, speeding up reactions. This is how enzymes catalyze reactions, making them occur faster.

• Enzymes do not change whether a reaction is exergonic or endergonic; they only reduce the activation energy.

• Enzymes remain unchanged by the reaction they catalyze, allowing them to be used repeatedly.

Factors Affecting Enzyme Activity

• Substrate Concentration: Enzyme activity increases with substrate concentration up to a point, after which it plateaus due to enzyme saturation.

• Enzymes are proteins and can denature outside specific temperature ranges, losing their structure and function.

• Optimal pH: Most enzymes have an optimal pH for function (e.g., pepsin in the stomach, trypsin in the intestine), reflecting the conditions in their native environments.

• Temperature: Raising temperature generally speeds up reactions, but extremes can cause denaturation. Enzymes have an optimal temperature range for activity.

Cofactors and Coenzymes

• Cells regulate enzymes by controlling their presence, concentration, activation, or deactivation to maintain metabolic balance.

• Cofactors: **Essential ions or small substances that enhance or are essential